Rubella seroprevalence among pregnant women in Burkina Faso

Background Despite the serious consequences of rubella infection during early pregnancy, very little is known about the rubella seroprevalence in a number of African countries including Burkina Faso. Methods Between December 2007 and March 2008 serum samples were collected from 341 pregnant women in Bobo (n = 132, urban area) and Houndé (n = 209, rural area) and were tested for rubella-specific IgG antibodies with a commercial ELISA kit. Results An overall seropositivity rate of 95.0% (324/341) was found, with a higher percentage in the urban population and in the oldest age group. Considering an antibody titer of at least 10 International Units per ml as protective, the overall immunity rate in the cohort of pregnant women was 93.3% (318/341). Conclusions The high overall seropositivity rate in the absence of routine immunization suggests a continuous transmission of endemic rubella virus in Burkina Faso, posing a threat to non-immune pregnant women.


Background
Rubella is normally a self-limiting febrile illness without significant long term morbidity [1]. Infection during pregnancy, however, may lead to miscarriage, fetal death or the birth of an infant with congenital rubella syndrome (CRS) [2]. It is estimated that worldwide more than 100 000 children with CRS are born each year [3]. Although the rubella seroprevalence among women in child-bearing age has been studied in several African countries [4][5][6][7][8][9], only one report from the Upper Volta region in Burkina Faso dating back to 1982 [10] and a recent study comprising only 100 women from a single location [11] are currently available to estimate the CRS risk in this country. Rubella vaccination is not included in the national immunization schedule in Burkina Faso and only few doses of vaccine are applied in the private sector. The aim of this study was to determine how many pregnant women are at risk of primary infection with rubella in a rural and urban area in the region of Bobo-Dioulasso, Burkina Faso.

Methods
Venous blood samples were collected between December 2007 and March 2008 from 341 pregnant women in health centers in Bobo (n = 132, urban area) and Houndé (n = 209, rural area). The women's age ranged from 16 to 42 years (mean: 25.7 ± 5.8 years), with more than half (58.9%) of them being 20-29 years old (Table 1). Informed consent to participate in the study was obtained from all women and a questionnaire comprising data on educational, marital, and pregnancy status, monthly income and history of previous exanthematous diseases was completed for each participant. All samples were transported to the laboratory on the day of collection for serum extraction and subsequent storage at -20°C.
The serum samples were screened manually for rubellaspecific IgG antibodies using a commercial ELISA test (Enzygnost Anti-Rubella-Virus/IgG, Siemens, Germany). All equivocal samples were retested and if the result was confirmed, the sample was classified as equivocal, otherwise as positive or negative.
Statistical data analysis was done using the statistical package for social science software (SPSS, version 15, Chicago Incorporation), Data Analysis and Statistical Software (Stata, version 10, StataCorp LP, Texas, USA) and Microsoft Office Excel 2004. Tests of significance were conducted using the chi-square test at a significance level of 0.05. The study protocol was approved by the Institutional Ethics Committee of Centre Muraz.

Discussion
This study provides for the first time rubella seroprevalence data for an urban and a rural setting in the western part of Burkina Faso. Though the study is limited to 341 pregnant women from two locations, the high overall immunity rate suggests a well-supported and continuous transmission of endemic rubella virus in the country. As a result most women become infected before reaching childbearing age. Nevertheless, the number of protected women continues to increase from 89.1% below the age of 20 to more than 95% in the 20-29 year olds, the age at which nearly 60% of the women gave birth. Beyond the age of 40 all women have antibodies against rubella. Since rubella vaccine is rarely given to adults in Burkina, our results suggest that a sizable number of women become infected during child-bearing age. In a previous report from Ouagadougou in central Burkina, an overall seroprevalence of 77% was found among pregnant women and the seropositivity rates seemed to decrease with age from 85% in the 18-25 year olds to 65% in the 36-50 year olds [11]. Nevertheless, the overall results indicate that a considerable proportion of pregnant women in Burkina are at risk of primary infection with rubella virus. In Burkina vaccination against rubella is provided by the private sector to a limited number of children. This low-coverage immunization may lead to a reduced circulation of rubella virus followed by a higher age of first exposure and therefore increased risk of CRS [13]. Based on the overall immunity rate found in this study and an estimated birth rate for Burkina Faso of 43.2 births/ 1000 population [14], we calculated that per year about 50000 pregnant women are at risk of rubella infection. Without any information on rubella incidence in Burkina Faso, reliable estimates of CRS case numbers are impossible, but considering that up to 90% of primary infections during the first 8-10 weeks of gestation may lead to fetal defects [12], the number of CRS cases is potentially quite high. Prevention of CRS is the main aim of rubella vaccination. Thus a rubella vaccination strategy in Burkina needs to ensure high enough vaccination coverage rates (≥ 80%) to interrupt indigenous virus circulation [12]. If this cannot be assured in a sustainable way the number of CRS cases may even further increase [13]. Similarly, vaccination in the private sector which does not reach high enough coverage can lead to more CRS cases [15].

Conclusions
In conclusion, this study presents for the first time rubella seroprevalence data for an urban and a rural setting in the western part of Burkina Faso and although the study is restricted to the Bobo-Dioulasso region, the high seropositivity in the absence of routine vaccination suggests a continuous transmission of endemic rubella virus in Burkina Faso. While most women acquire seropositivity before reaching childbearing age, the virus represents a considerable threat to non-immune pregnant women.